This invention relates to high-intensity-discharge lamps and a method for applying coatings comprising boron oxide to the surface of a vitreous silica substrate.
Mercury metal-halide high-intensity-discharge (HID) lamps are well known and have found wide application in area lighting and stadium lighting. In one embodiment of such lamps, the discharge-sustaining filling comprises mercury, sodium halide, and scandium halide, as taught in U.S. Pat. No. 3,407,327 dated Oct. 22, 1968 to Koury et al. When sodium halides are utilized in conjunction with a vitreous silica arc tube, there is a tendency for the sodium ions to migrate through the vitreous silica arc tube during prolonged operation of such lamps. Various constructions have been devised to minimize or inhibit such sodium ion migration and while these have achieved some degree of success, sodium ion migration through the vitreous silica arc tube remains a continuing problem.
U.S. Pat. No. 3,390,298 dated June 25, 1968 to Werner discloses vitreous silica lamp envelopes intended to operate at very high temperatures and which have provided on the interior surface thereof a coating comprising boron oxide, in order to inhibit so-called devitrification of the vitreous silica. Such coatings have been applied by wetting the inner surface of the envelope with a solution of one part glycerin and three parts trimethylborate, then heating the envelope and applied coating to a temperature of 1200.degree. C. to 1300.degree. C. for two hours in order to form a boric oxide-silica glass layer. The usual HID lamp is not, and frequently cannot, be heated to such an extreme temperature during its processing.
The problems encountered with the extra high-temperature heating step as taught in U.S. Pat. No. 3,390,298 are recognized in U.S. Pat. No. 3,679,385 dated July 25, 1972 to Sneft, which discloses an improved coating technique wherein inert gas in which the borating material, such as trimethylborate, is entrained and is then deposited on the inner surface of very heavy vitreous silica tubing prior to formation of the bulbous portion of the envelope. When the bulbous portion of the envelope is formed, the extremely high temperature required to soften vitreous silica to enable it to be formed also cause the deposited borating layer to react with the vitreous silica substrate.